This invention relates to the field of microcapsules having a nucleus material encased within a double shell material, where an inner shell comprises polymeric material and an outer shell comprises cross-linked complex colloid material, particularly formaldehyde capsules encased within gelatin, with glutaraldehyde having been used as a cross-linker for the gelatin. The invention also contemplates use of such microcapsules in printing processes, resulting in good printability and improved efficiency in the amount of microcapsule material used to obtain a specified level of print definition on paper stock.
As used herein, a microcapsule is defined as having a diameter of about 1 micron to about 300 microns, preferably about 5-100 microns. Microcapsules have many applications, such as in manufacture of pharmaceuticals, pesticides, paints, adhesives, and many other chemical products. Microcapsules are especially useful where it is desired to provide controlled release of an enclosed and contained nucleus material, namely the substance being encapsulated. In one example of controlled release, the product known as xe2x80x9ccarbonless paperxe2x80x9d is made by providing at least one component of a two-component colorant as the nucleus material in such microcapsules.
When the two components are mixed, such as when the encapsulated component is released from the capsule, the color-producing material is released and thus enabled to provide the desired coloration. In such carbonless paper, a layer of capsules of one or both components of a 2-component color-generating system may be coated onto a surface of paper or other fibrous web or sheet, or onto facing surfaces of facing sheets of paper or other fibrous web or sheet. When the capsules are broken, such as by pressure on the paper, the encapsulated colorant component is released, whereby the color-producing activity is enabled.
In one well known process, known by the term coacervation, the microcapsules can comprise an e.g. oil-containing nucleus material, and oil-impermeable shells formed of gelled complex polymerized materials. Principles of coacervation are taught by e.g. U.S. Pat. No. 2,800,457, and in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 13, John Wiley and Sons, 1967, Chapter on Microencapsulation, pages 436-456.
Coacervation comprises the phenomenon of phase separation in certain liquid polymer compositions leading to formation of two or more liquid phases, and deposition of polymerizable liquid shell material onto dispersed particles of liquid nucleus material. The cooperative formation of disperse particles, each having two such distinctly different liquid phases, distinguishes coacervation from precipitation of polymer solute in solid form in a liquid solvent. Coacervation can be activated by e.g. adjusting pH of the mixture. Both the gellable shell material and the nucleus material must be ionizable; and the combination of nucleus material and shell material must exist in the mixture, under certain conditions, with opposite ionic charges simultaneously existing on respective ones of the nucleus material and the shell material such that the respective particles of nucleus material and shell material are attracted to each other. Such opposite charges can be achieved by proper selection of the nucleus material and the shell material, and by adjusting pH or other physical property where one or both of the shell material and nucleus material are amphoteric, so as to effect polarity change. After the microcapsules are formed, the gelled or otherwise polymerized shell material can be hardened, optionally separated from the e.g. solvent liquid, dried, and if desired, comminuted to a desired particle size.
A liquid carrier such as oil can be used as the primary nucleus material, to carry one or more dispersed acting materials, either solid or liquid acting materials, including materials which can evaporate or degrade due to exposure to air. Additionally the carrier, itself, can be the material of interest in the nucleus, such as, for example, a perfume or marking fluid.
This invention relates to processes for en masse manufacturing of minute capsules, referred to herein as microcapsules, in a liquid manufacturing medium. The processes of the invention involve liquid-liquid phase separation of a relatively concentrated solution of polymeric material to be used in the formation of shells for the minute capsules. The processes of this invention involve, for example, the polymerization of urea and formaldehyde, monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea, melamine formaldehyde, monomeric or low molecular weight polymers of methylol melamine or methylated methylol melamine, in an aqueous vehicle wherein the reaction is conducted in the presence of certain acrylic acid-alkyl acrylate copolymers.
The sizes of microcapsules can suitably be chosen depending upon the expected end use. Where microcapsules are employed in e.g. pressure sensitive recording sheets, preferred microcapsule size is about 5 microns to about 30 microns in order to enable creating sharply defined images using the chromogenic nucleus material contained in such microcapsules. Where the microcapsules are to be coated onto a fibrous or otherwise porous sheet or web, such individual microcapsules may be so small as to become significantly recessed below the surface of the sheet or web, and accordingly cushioned from a crushing force directed toward the surface of such sheet or web. As a result, a normal activation pressure on the sheet or web is ineffective to rupture and thus activate, such individual microcapsules. Such recess of the microcapsule into the web or sheet can be overcome by employing relatively larger size microcapsules, but the resulting images created using such microcapsules exhibit relatively less clarity and sharpness of edge definition because of the larger size microcapsules. Yet a larger size particle is highly desirable in order to retain the particle at the surface of the sheet where such particle can more readily be broken by mechanical force applied at the surface of the sheet. Such larger size particle which can be retained at the surface of the sheet, while providing excellent image definition, is achieved in the invention by providing aggregates of the desirably small size microcapsules.
A method of encapsulation by in situ polymerization including a reaction between urea and formaldehyde or polycondensation of monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea in an aqueous vehicle conducted in the presence of negatively-charged, carboxyl-substituted linear aliphatic hydrocarbon polyelectrolyte material dissolved in the vehicle, is disclosed in U.S. Pat. Nos. 4,001,140; 4,087,376; and 4,089,802.
A method of encapsulating by in situ polymerization, including a reaction between melamine and formaldehyde or polycondensation of monomeric or low molecular weight polymers of methylol melamine or etherified methylol melamine in an aqueous vehicle conducted in the presence of negatively-charged, carboxyl-substituted, linear aliphatic hydrocarbon polyelectrolyte material dissolved in the vehicle, is disclosed in U.S. Pat. No. 4,100,103.
A method of encapsulating by polymerizing urea and formaldehyde in the presence of gum arabic is disclosed in U.S. Pat. No. 4,221,710. This patent further discloses that anionic high molecular weight electrolytes can also be employed with the gum arabic. Examples of the anionic high molecular weight electrolytes include acrylic acid copolymers and under specific examples of acrylic acid copolymers are listed copolymers of alkyl acrylates and acrylic acid including methyl acrylate-acrylic acid copolymers, ethyl acrylate-acrylic acid copolymers, butyl acrylate-acrylic acid copolymers and octyl acrylate-acrylic acid copolymers.
An exemplary method of preparing microcapsules by polymerizing urea and formaldehyde in the presence of an anionic polyelectrolyte and an ammonium salt of an acid is disclosed in U.S. Pat. Nos. 4,251,386 and 4,356,109. Examples of the anionic polyelectrolytes include copolymers of acrylic acid. Specific examples of acrylic acid copolymers are copolymers of alkyl acrylates and acrylic acid including methyl acrylate-acrylic acid, ethyl acrylate-acrylic acid, butyl acrylate-acrylic acid and octyl acrylate-acrylic acid copolymers.
The most widespread use of microcapsules to date has been in certain kinds of pressure-sensitive copying systems. In one such system, disclosed in U.S. Pat. No. 2,730,456 and commonly known as manifold record material, an upper sheet is coated on its lower surface with microcapsules containing a solution of a colorless chromogenic material (hereinafter referred to as a coated back sheet or CB sheet), and a lower sheet is coated on its upper surface with a color developing coreactant material, e.g. an acidic clay, a phenolic resin or certain organic salts (hereinafter referred to as a coated front of CF sheet). For implementations which require more than two plies in the record material, a number of intermediate sheets are also provided, each of which is coated on e.g. its lower surface with microcapsules and on its upper surface with acidic, color-developing material. Pressure exerted on the sheets by writing or typing ruptures the microcapsules, thereby releasing the chromogenic material solution onto the coreactant material on the next lower sheet and giving rise to a chemical reaction which develops the color of the chromogenic material.
In another system, known as a self-contained system and disclosed in U.S. Pat. Nos. 2,730,457 and 4,197,346, microcapsules containing a chromogenic material solution and a coreactant material are coated onto a surface of a sheet of paper in combination with a cooperating coating of a co-reactant material on the same sheet of paper. Pressure exerted on the sheet by writing or typing causes the capsules to rupture and release the chromogenic material, which then reacts with the coreactant material on the sheet to produce a color.
Microcapsules for use in the above-described pressure-sensitive copying systems must satisfy certain property requirements so as to produce a desirable copying system. Some of these properties are capsule strength, particle size, particle size distribution, and shell permeability.
The processes according to U.S. Pat. Nos. 4,001,140; 4,087,376; 4,089,802; and 4,100,103 have been successfully used to encapsulate liquid compositions of chromogenic materials for use in pressure-sensitive copying papers using certain materials as system modifiers to facilitate the encapsulation process. Of the carboxyl group system modifiers disclosed in the patents, hydrolyzed maleic anhydride copolymers are preferred. Among the hydrolyzed maleic anhydride copolymers, poly(ethylene-co-maleic anhydride) (hereinafter referred to as EMA) is typically preferred because of the balance of properties which EMA provides to the encapsulation processes and the resulting microcapsules.
The cost of EMA, relative to other eligible system modifiers is a premium, whereby the cost of microcapsules, manufactured by processes in which EMA constitutes the system modifier, is a premium cost. Because of cost and availability considerations, poly(acrylic acid) (hereinafter referred to as PAA), is an acceptable substitute for EMA as the system modifier. While microcapsules made using processes according to U.S. Pat. Nos. 4,001,140; 4,087,376; 4,089,802; and 4,100,103, in which PAA constitutes the system modifier, are of commercial quality for use in pressure-sensitive copying paper, such microcapsules do not possess an optimum balance of properties corresponding to the properties obtained when EMA is utilized.
One function of the system modifier in the patents is to take an active part in the control or moderation of the polymerization reaction of the starting materials used to form the condensation polymer which makes up the resulting capsule shells.
Another function of the system modifier in the patents is to act as an emulsifying agent to promote and maintain the separation of the individual droplets of the intended capsule nucleus material in the aqueous manufacturing vehicle.
When PAA is utilized as the system modifier, emulsification of the intended capsule nucleus material requires more energy input and time and produces a less desirable size distribution than when EMA is employed.
The less desirable emulsifying capability of PAA can be offset in the case of the process of U.S. Pat. No. 4,100,103 by mixing in, prior to emulsification, the starting materials (e.g. methylated methylol melamine) employed in the in situ polymerization reaction to form the condensation polymer which makes up the resulting capsule shells. The presence of methylated methylol melamine or a low molecular weight polymer thereof (hereinafter referred to as MMM) during the intended nucleus material emulsification step, can result in the premature polymerization of the MMM. The tendency of the MMM to prematurely react under these circumstances is reduced by raising the pH of the PAA-MMM solution to the highest level at which emulsification of the intended nucleus material can be obtained.
Once a satisfactory intended nucleus material emulsion is obtained, the pH of the emulsion is reduced in order to obtain the deposition of satisfactory capsule shell material about the nucleus material particles in a reasonable amount of time. This process is further modified by the addition of certain salts as disclosed in U.S. Pat. No. 4,444,699 of Donald E. Hayford. PAA can also be used as the system modifier optionally in combination with polystyrene sulfonic acid or a salt thereof in which the polystyrene sulfonic acid is present.
While the coacervation process has many advantages, some disadvantages are associated with conventional coacervation processes. For example, it can be difficult to achieve a high level of control of the sizes and size distribution or size range within a population of the microcapsules. Inadequate and/or inconsistent agitation of the mixture can produce capsules which are larger than a maximum desired size suitable for the contemplated application. Such larger capsules produce color indications of undesirably reduced definition and clarity.
The particle size of microcapsules required for good clarity and definition of a color developed image can be necessarily so small that such particles become embedded in the e.g. paper sheet onto which such microcapsules are coated, such that the structure of the paper sheet cushions the microcapsules from applied mechanical force intended to fracture the microcapsules and thus release the nucleus material.
Conventional liquid-phase methods of making capsules, such as the coacervation process, can produce acceptable quality encapsulated product in a limited number of combinations of shell material and nucleus material without deleterious affect of the nucleus material on the shell material so as to result in an undesirably high level of permeation of the nucleus material through the shell material.
Another disadvantage of microcapsules made using conventional coacervation processes is that the nucleus material, or combination of nucleus materials, can chemically attack the shell material over time, thereby negating the value of such microcapsules.
It is an object of the present invention to provide improved microcapsules having dual shells, wherein the inner shell is a polymer material, preferably a polymer polymerized from polar precursor materials, such as various of the formaldehyde or other aldehyde-derived polymers, for example a melamine-formaldehyde or derivative, or a urea formaldehyde or derivative, and the outer shell is a combination of a polymerized or otherwise coalesced and consolidated solid material, preferably gelatin or other complex colloid, along with use of a suitable cross-linking material, such as an aldehyde, preferably glutaraldehyde.
It is an object of the present invention to provide microcapsule aggregates having dual shells, wherein the inner shell about each nucleus is a polymer material, preferably a polymer polymerized from polar pre-polymer materials, and the outer shell is a combination of a polymerized or otherwise coalesced and consolidated solid material, preferably gelatin, along with use of a suitable cross-linking material, such as an aldehyde, preferably glutaraldehyde.
It is a more specific object of the invention to provide such microcapsules in a given population preferably having an average size correlating to a spherical diameter of about 5 microns to about 50 microns, and a narrow size distribution range having a standard deviation of up to about plus or minus 25 percent.
It is a further object to provide such double shell microcapsules in aggregate form wherein a population of such aggregates has an average particle size correlating to a spherical diameter of about 15 microns to about 200 microns, with a narrow size distribution range having a standard deviation of up to about plus or minus 25 percent.
Another object of the invention is to provide paper stock having equivalent or relatively improved printability characteristics relative to sizes of the individual discrete particles while utilizing a relatively reduced loading of such microcapsules or microcapsule aggregates of the present invention.
A further object of the invention is to provide methods for manufacturing dual shell microcapsules having a desired size and a narrow size distribution range, where the inner shell is a polymer material derived from a carboxylic monomer, preferably an aldehyde-derived resin, and the outer shell is a polymerized and consolidated, e.g. cross-linked, gelatin, preferably cross-linked with an aldehyde, more preferably cross-linked with glutaraldehyde.
It is yet another object of this invention to provide a process for loading a paper stock by loading such paper stock with a reduced mass of nucleus material, e.g. dye or dye precursor material in microencapsulated form while retaining equivalent or improved activation properties of the encapsulated nucleus material, e.g. printability characteristics, including sharpness of image definition, to the paper stock.
The present invention provides improved microcapsules and microcapsule aggregates, and processes for making such microcapsules and microcapsule aggregates. The invention further provides methods for more efficiently utilizing microcapsules in paper stock while retaining desired e.g. printing properties, wherein the paper stock has acceptable e.g. sharpness of image printing qualities in combination with a reduced microcapsule loading.
The processes of the invention involve deposition of gelatin or a gelatin derivative or other complex colloid, about a previously-formed microcapsule, for example having a single layer or multiple layer shell, including about a previously-formed capsule shell. In the alternative, the processes of the invention involve formation and polymerization of a first inner capsule shell about a droplet of nucleus material with formation and polymerization of a second outer capsule shell, outwardly of and deposited about the inner capsule shell, either in partial combination with formation of the inner shell or shortly after formation of the inner shell and optionally under processing conditions similar to the conditions which resulted in formation of the inner shell.
The processes of the invention involve controlled agglomeration of the capsules into coalesced, controlled-size aggregates having inner and outer shells wherein the outer shell material serves as an outer skin on such aggregates.
To overcome the problems outlined above, novel microcapsules and microcapsule aggregates have been developed in this invention, each microcapsule having a nucleus material, each microcapsule having an internal shell of solid-phase polymerized polymer material and each microcapsule, or aggregate of microcapsules, having an external shell of gelled gelatin, gelatin derivative, or other complex colloid.
The internal and external shells of a respective microcapsule or microcapsule aggregate are adherent in combination and in general to each other to form a two-layer shell around the nucleus material. The inner shell surrounds the nucleus material. The outer shell generally surrounds the inner shell.
Where multiple microcapsules form respective aggregate particles, the outer shell material surrounds the individual inner shells and the respective microcapsules in a given aggregate particle collectively adhere to each other.
In the alternative, an individual microcapsule can be thought of as comprising the nucleus material and an inner shell, with the inner shells of adjacent ones of such microcapsules touching each other or being in closely-spaced proximity with each other, and wherein a collection of such individual microcapsules being encompassed by and/or surrounded by a skin of the outer shell material. In such case, the inner shells of the respective microcapsules can be touching each other, or can be spaced in close proximity with each other with a relatively thinner layer of outer shell material disposed between outer surfaces of the inner shells of adjacent ones of the microcapsules. Such consideration of relatively thinner layers of outer shell material between the inner shells of adjacent microcapsules is addressed at the loci of closest approach of the respective adjacent microcapsules, and is compared to an average thickness of the outer shell material on the outside surface of the aggregate particle where the outer shell material is underlain by an inner shell of a microcapsule at the surface of the aggregate. In this regard, shell thickness is defined along a line extending perpendicular to the outer surface of the inner shell material, and through a central locus, e.g. center, of the microcapsule.
In view of the opportunity to select two different shell materials for the shells, each shell material can provide less than all properties required of the shell. Accordingly, such double shell structure can be stronger, less permeable to the nucleus material, and more resistant to chemical attack while employing a wider range of shell materials than can be employed in single-shell microcapsules. Such double shell structure can thus offer two shell material selections, instead of just one, to more assuredly physically and chemically contain the nucleus material, and to control strength, shell thickness, capsule or capsule aggregate size, capsule or capsule aggregate durability, and the like.
Accordingly, the invention comprehends a population of pressure sensitive microcapsule aggregate particles. Respective aggregate particles in the population comprise a plurality of inner shells polymerized from polar pre-polymer materials, the inner shells extending about, and defining, closed central chambers, said inner shells being disposed in close proximity to each other in a given aggregate particle in an arrangement of said inner shells. Each of the closed central chambers contains liquid chromogenic composition. Outer shell material defines an outer shell collectively enveloping the plurality of inner shells in a given aggregate particle and thereby collectively joining the inner shells together as the aggregate particle.
In some embodiments, the outer shell material is selected from the group consisting of gelatin, gelatin derivatives, and other complex colloids capable of forming the outer shell.
In some embodiments, the outer shell material surrounds substantially the entirety of each inner shell.
The outer shell preferably comprises cross-linked gelatin and/or gum arabic, and the inner shell preferably comprises an aldehyde derivative.
The aggregate particles preferably have effective diameters of about 20 microns to about 35 microns.
The outer shell material preferably comprises about 2 weight percent to about 10 weight percent, more preferably about 3 weight percent to about 8 weight percent of the composition of said aggregate particles.
Another aspect of the invention comprises pressure sensitive copying material wherein such pressure sensitive copying material comprises a substrate fibrous sheet material for receiving a coating of marking material, and a marking material coated on and affixed to the substrate fibrous sheet material. The marking material comprises pressure-sensitive microcapsule aggregate particles. Each aggregate particle contains a liquid chromogenic composition enclosed in a plurality of inner shells polymerized from polar pre-polymer materials. Each inner shell extends about and encloses a droplet of the liquid chromogenic composition. The inner shells are disposed in close proximity to each other in an array of the inner shells. An outer shell collectively encompasses the inner shells thus to define an outer surface of the respective aggregate particle.
The invention further comprehends a method of making aggregate particles containing droplets of chromogenic material wherein a respective aggregate particle comprises multiple droplets of such chromogenic material, each contained in a microcapsule defined by an inner shell. An outer shell encompasses the entirety of the aggregate particle and thereby collectively contains the enclosed inner shells and droplets. The method comprises forming microcapsules from polar pre-polymer materials in an aqueous processing mixture, each microcapsule comprising an inner shell and a droplet of chromogenic material contained therein. The method further includes, after formation of the inner shell, adding an outer shell material comprising gelatin to the reaction mixture and thereby agglomerating the inner shells into aggregate particles wherein gelatin forms an outer shell coating encompassing each respective aggregate particle.
The method can include adding the gelatin in a quantity of about 2 weight percent to about 10 weight percent of the quantity of the combination of the gelatin, the inner shells, and the droplets of chromogenic material.
The method preferably includes forming the inner shells using a composition rich in aldehyde.
Preferred embodiments of the method include mixing the mixture and treating the mixture with an agglomeration control agent so as to obtain aggregates having average effective diameters of about 20 microns to about 35 microns.
The invention also comprehends a method of making aggregate particles containing droplets of chromogenic material wherein a respective aggregate particle comprises multiple droplets of such chromogenic material, each contained in a microcapsule defined by an inner shell. An outer shell encompasses the entirety of the aggregate particle and thereby collectively encompasses the enclosed inner shells and droplets.
The method comprises initiating formation of the microcapsules from polar pre-polymer material, in an aqueous processing mixture such that initiation of each such microcapsule initiates development of an inner shell about a droplet of the chromogenic material; and after initiating formation of the microcapsules, and before reaching an end point of such development of the microcapsules, adding to the aqueous mixture an outer shell material which deposits on the inner shells thus attenuating deposition of inner shell material from the liquid mixture onto the developing inner shells, and wherein the outer shell material agglomerates the developing microcapsules together and forms aggregate particles thereof and forms an outer shell coating encompassing each respective aggregate particle.